Apparatus for making piezoelectric crystals



H, F. FRUTH July 16, 1946.

APPARATUS FOR MAKING PIEZOELECTRIG CRYSTALS Filed April 24, 1944 INV NTOR, F 222272,

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Patented July 16, 1946 APPARATUS FOR MAKING PIEZOELECTRIC CRYSTALS Hal F. Fruth, Chicago, Ill., assignor to Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application April 24, 1944, Serial No. 532,430

3 Claims. (Cl. 51-124) The present invention relates to improved apparatus for manufacturing piezoelectric crystals, and more particularly, to improved apparatus for grinding the faces of piezoelectric crystals to impart predetermined frequency characteristics thereto. a

In the manufacture of piezoelectric crystals, such, for example, as quartz crystals adapted for use in communication circuits, and more particularly, for use in crystal microphones, radio transmitting and receiving systems, and the like, the crystal blanks are first cut from the crystal stock and are then ground to the dimensions required to provide the desired frequency and activity characteristics. More particularly, the usual crystal blank is cut in the form of a waferlike rectangular piece, having dimensions slightly larger than the desired dimensions, is then reduced in size to approach the desired rectangular dimensions by the use of lap-and edge grinding apparatus, and finally is reduced to the exact desired dimensions by finish grinding operations.

It is an object of the present invention to provide improved grinding apparatus for rapidly grinding the faces of a crystal to render the faces substantially flat and substantially parallel, without the production of deep scratches or other surface irregularities therein.

It is another object of the invention to provide improved lap grinding apparatus which may be easily loaded with a plurality of crystal blanks and in which the contact pressures used in grinding the different crystals may be determined on an individual crystal basis.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood by reference to the following specification taken in connection with the accompanying drawing, in which:

Fig. lis atop perspective view illustrating improved lap grinding apparatus characterized by certain features of the present invention and utilized in part in the practice of the present improved methods;

Fig. 2 is a side View of the apparatus shown in Fig. 1'; and

imate desired dimensions, all three dimensions of each blank being considerably greater than the desired dimensions in order to permit the grinding operations to be carried out. The rough cut crystal blanks are next placed in the lap grinding apparatus shown in Figs. 1, 2 and 3 of the drawing, and subjected to a lap grinding operation for a definite interval of time sufficient to insure that one face of each crystal will be ground exactly fiat. After one face of each crystal has been thus ground to impart a flat contour thereto, belt grinding apparatus is used to pressure grind the opposite face of each crystal for the purpose of reducing the thickness of the crystal. Following this operation, which is utilized to reduce the thickness of each crystal more nearly.

, to approach the desired thickness with considerablegrindin-g rapidity, the crystal blanks are returned to the lap grinding apparatus and both face surfaces of each crystal are ground in this apparatus in order to remove deep scratches and pits from the crystal faces and to further reduce the thickness of each crystal more nearly to appreach the desired thickness. In this regard, it

Fig. 3 is a fragmentary sectional view illuswill be understood that the thickness of each crystal determines its resonant frequency, the whole object of the face grinding step being that of reducing the thickness'of the crystal to the exact desired thickness in order to impart a predetermined resonant frequency to the crystal, while at the same time obtaining a highly polishedand scratch-free surface. In other words, the thickness of each crystal may be measured in terms of the resonant frequencyof the crystal,

During the third described step of lap grinding the crystal faces, the crystals are ground until the resonant frequency of eachcrystal is from ten to sixty kilocycles lower than the particular desired resonant frequency, in preparation for finish grinding to the exact desired frequency.

Referring now more particularly to the structural arrangement of the lap grinding apparatus illustrated in Figs. 1, 2 and 3 of the drawing, this apparatus comprises a circular plate 10 whichis rigidly mounted at the upper end of a shaft 12 for rotation about its center, and is provided with a fiat upper abrading surface lllbr The shaft I2 is iournaled for rotation in bearing members [3 which are mounted upon opposite sides of a supporting platform I I. This platform is held in ele-' vated position above a base, not shown, by means of side members, one of which is indicated at I la,

such that the top surfaces thereof are exactly flush with the top abrading surface of the plate In, by means of supporting posts-I6, which-are anchored to the top of the platform 'II in any suitable manner.

Circular crystal holding plates I8 and [9, each provided with a plurality *of -crystal receiving pockets 22 and 23, are utilized to support the crystals in engagement with the top abrasive surface x More specifically, these lb of the plate I0. holders are rotatably supported at the ends Fla and Nb of a bifurcated arm I11, 'pivotpinsilland 2| being utilized to rotatably mount the ,plates 18 and I9 at the ends of the two arm bifurcations." The arm I! is pivotally mounted at the upper endo'f a post z l extending upright from the platform "I I and rigidly anchoredthereto'byimeans of mountin screws 25; For the purpose of pivoting the'arm 1 back and'forth through a predetermined angle, thereby to sweep the crystal holders I8 and I9 back and forth through a limited are which overlies the abrasive upper surface Iiib of the plate l9, this arm is connected to a rotatable.

crankplate 26 through a pinand'slot connection which comprises-spin 2 9 extending upwardly from the plate 26 within :a slot 2% formed longitudinally of the'arm. The plate 26 isset screw mounted upon the upper end of a second rotatable shaft 2 I whichis'journaled intermediate'theends thereofwithinbearing members '28 disposed upon opposite sides of the platform II, and is driven at a-slow-spejedeither'through a, speed reducing gear box' actuated by the driving motorfor the shaft I2 or-by a secondslow speed electric motor directly "connected to the shaft 2 and carried by thebase located at the bottom ends of the side members Ila. 1

r'ie'fiy to consider the operation of the described lapping apparatus, it may be pointed out that before rotation of the shaft l'lto-drivethe abradin'g platelo'and rotation of thesh'af't 21 to pivot the arm 'II back and forth are initiated, a crystal 9' is placed in each or a portion 'of' the pockets "22 and 23 respectively providedin the two holders I8 and i9. Depending upon the position of'the' arm Il, each crystal is disposed facedo'wnwardly'against the upper-surface of-one *of the twostationary members 1'4 and I5 or theu-pper abradingsurface I 'li b'of the plate "I0. In this regard it'is noted that the abradingjsurface I 06 of thep'late I'll is ground perfectlyilat,that-theupp'er surfaces of the ,twoimembers' 14 and l'5 'are likewise perfectly flat and are exactly flush with the surface I'llb of the plate l d. 'Weight's in the form of liquid filled rubber cups 316 are utilized to es tablish the desired contact 'pressurebe'tweehthe bottom-surface of each crystal 9 and the upper surface of the memberjfll, I4or f5 against which the, particular crystalbears; More specifically, each partially filled liquid receivingcup indetermin'es the'contajct pressurebetweenthe lower face of the crystal which-it Weightsjand the upper ii'eiiiblebottorhwall 39binorderthatadistributed pressure "over the entire 'upper surface of the as;

4 sociated crystal 9 may be produced by the liquid 3| with which the cup is partially filled. Any liquid, such, for example, as mercu y, having great weight per unit volume, may be utilized in filling the cups 3|] to the level required to establish the desired contact pressure. After the pockets of the holders I8 and I9 have been loaded in the manner just explained the 'abrading surface Illb of the plate I!) may be sprinkled with diamond dust or another loose abrasive of suitable fineness,

followin which operation of the driving motor or motorsto-rotate the two shafts l2 and 21 may be initiated.

In :response to rotation of the shaft 21 the arm l:'l isupivoted:back and forth about the pivot pin zl'through a limited angle by means of the pimand slotconnection between this arm and the plate 26. As a result, the crystal holders I8 and I9 are moved back and forth across the center of the plate I9, so that peripheral portions thereof alternately overlap the two stationary members I4 and 'lii. Incident to this movementof the crystal holders, each crystal is moved across the abrading surface I91; from one sided the center of rotation of the plate H! to "the'other, with the result that the lower surfaces of 'the crystals are rapidly abraded. During movement of the arm I! fromone limit'of its range of move ment to the other, each oft-he "two crystal holders I8 and I9 is rotated relativeto the plate IE3 in order to' change the position of each crystal relative to the abrasivesurface I06 of the plate It]. Thus assuming thatthe-plate I'll isrotating in a counterclockwise direction, as viewed in Fig. l' of the drawing, and that; [the crystal holder I9, for example, occupies the position shown in full lines in "this figure of the drawing, the frictional engagement between the abrading surface lllb of the late- It and the-engaging faces of the crystals within the pockets 2'3 of the holder causes the holder to berotated' in a counter clockwise direction, suc'h'thatthe positions of the crystals relative to-the abrading surface are continuously -'ch'anged.- Y As the holder I9 is moved from this position to a position wherein the "center. of the holder substantiallyoverlies the center of the -plate HI,- the retatienal forces acting upon this holder are more effectively utilized to cut down the degree of slip between crease-in the speedof rotationbftheholderd as the 'helder is moved tower the edge of the plate 1c is accompanied by increase in"the linearspeed at which the abrasive' surface 'l'flb underlyin the crystal faces is--'-meved relative to these faces; Thus it'will be appairerit' that the rate of abrasion is "increased as the liolder l9 is moved towards the edge of'the plate *Ill; filfir'ing continued movement of the holder ['9 'in-the indicated direction, a point is reached fat which the edge portion of- -'the holder engages" the "edge of the stationary member 1 5:; When this occurs and du'e to the f'ricti'onal di'a'g between the engaging surfaces cf the parts it and 19, the'direction of ro 'fiihn of the holder I 9 about its pivot poi'nl'l2'lis reversed the'de'gre f ov'erlapbtween the-two ar wand "I 9 is increased, the resultant of the rotational forces-acting up0n the holder I9 is shifted in a direction to increase the speed of rotation of the holder l9 about its pivot point 2| in the reversed direction. Thus it will be apparent that the speed of rotation of the holder I9 is substantially constantly changed during movement of this holder from its illustrated full line position to its illustrated dotted line position. During movement of the arm I! in the reverse direction, the action is the reverse of that just described. As each crystal 9 within a pocket 23 of the holder I9 is moved to a position where the lower face thereof partially overlies and engages the abrading surface lllb and partially overlies and engages the upper surface of the stationary member I5, the frictional forces acting thereon tend to rotate the crystal within the pocket 23 in which it is disposed. Rotation of the crystal within its pocket is permitted due to the loose interfitting relationship between the side walls 30a of the rubber cup 3!) and the walls of the pocket. Thus each crystal is not only rotated about the axis 2| of the holder l9, but is also rotated about its own center during each movement of the arm I! from one limit of its range of pivotal movement to the other.

From the foregoing explanation it will be understood that due to the rotation of each crystal 9 in a plane parallel to the surfaces of the parts l4, l5 and ill, the edge sector of the crystal initially engaged by the rotating abrasive surface lub of the plate Ill is continuously changed in a more or less random manner with the result that the direction of cut across thecrystal face is continuously shifted at random. The same action of rotating each crystal'disposed within a pocket 22 of the holder I8 about two different centers of rotation during back and forth movement of this holder between the limits of its range of movement, is also obtained during the back and forth pivotal movement of the arm I]. The purpose of thus continuously changing the edge sector of each crystal which is initially engaged by the abrasive surface is that of preventing scratches from being formed in the crystal face being abraded, due to recurrent movement of the same abrasive particles along the same lines of abrasion across the crystal face. Thus it is possible with the described apparatus to obtain relatively rapid grinding of the crystal faces to a substantially flat contour Without producing scratches or other surface marks in the abraded crystal faces. Moreover, by the system of individual Weights comprising the cups 30 filled to the same or different liquid levels, either uniform or non-uniform grinding of the different crystals may be obtained, as desired.

While one embodiment of the invention has been described, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. Apparatus for face grinding piezoelectric crystals, comprising a circular abrading plate rotatable about its center and having a flat upper I I abrading surface, a stationary member having 7 a top surface flush with the upper surface of said plate and having an edge closely adjacent at least a portion of the periphery of said plate, an arm pivotally movable about a point removed from the axis of rotation of said surface, a

crystal holder pivotally connected to said arm for free rotation relativetosaid abrading plate,

and means for pivoting said arm back and forth through a limited angle which is so related to said abrading plate and stationary member'that said holder partially overlies said member each time said arm is moved to one limit of said angle. 7

2. Apparatus for face grinding piezoelectric crystals, comprising a circular abrading plate maintaining crystals face downward against said abrading surface, and means for pivoting said' arm back and forth through a limited angle which is so related to said abrading plate'that said holders are alternately moved to overlie different ones of said members as said arm is moved from either limit of said angle to the other.

3. Grinding apparatus comprising a circular abrading plate rotatable about its center and having a flat upper abrading surface, a stationary member having a, top surface flush with the upper surface of said plate and having an edge closely adjacent at least a portion of the periphery of said plate, a rotatable work holder for holding a work piece in a position offset with respect to the axis of rotation of said work holder and in contact with said abrading surface, means supporting said holder for free rotation about its axis of rotation, and means for so moving said supporting means that said work holder is moved transversely back and forth across said abrading surface and overlies the upper surface of said HAL F. FRUTTH. 

